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From: John Larkin
Newsgroups: sci.electronics.design
Subject: Re: My flyback experiment is on the web
Date: Thu, 17 Oct 2002 09:23:21 -0700
Organization: Posted via Supernews, http://www.supernews.com
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References: <3DAA0563.9040507@BOGUS.earthlink.net> <3daafdb7$0$11215$1b62eedf@news.euronet.nl> <3DAB8ABA.4030602@BOGUS.earthlink.net> <871568ec.0210150338.4dc52cb9@posting.google.com> <3DAC188B.7D82A78E@rica.net> <3DAC2D91.9020902@sandia.gov> <3DAC688E.9C870D83@rica.net>
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On Thu, 17 Oct 2002 03:34:30 -0000, rfwilson@intergate.nospam.bc.ca
(Bob Wilson) wrote:
>In article <3DAC688E.9C870D83@rica.net>, jpopelish@rica.net says...
>>
>>"Christopher R. Carlen" wrote:
>>>
>>> John Popelish wrote:
>>
>>> > I think these pulses are actually ringing in the gate drive line,
>>> > since they show very brief current ramps in the source current that
>>> > coincide with small positive gate voltage pulses. If the pull down is
>>> > brought a little negative, or stiffened I think these pulses would
>>> > disappear, and losses would go down.
>>> >
>>>
>>> See Fred Bartoli's response, he's explained it right. And my reply to
>>> Don Pearce.
>>>
>>> Good day!
>>
>>I disagree with you and Fred. The ringing should be a damped
>>sinusoid, not a series of fractional sine pulses if it were just
>>ringing.
>
>Quite correct. Rather obvious, actually, since ringing is simply a damped LC
>tank (usually parasitic) that is excited into resonance.
>
>Bob.
You can think of this as a ringing L-C in which the C is highly
nonlinear. If fact, this is what's going on. When the ring goes below
ground, which it wants to do, the substrate diode acts like a big
capacitor. So the 'spikes' are fast rings and the baseline zones are
slow rings. This isn't exactly right (the diode is nonlinear, and the
stored charge doesn't behave exactly like a capacitor) but
qualitatively this is about what's going on. A simple diode-L-C
circuit with proper initial conditions will ring just like this.
Once the ringing dies down enough that it no longer dips below ground,
you'll see the damped sine you expect. In this example, the next drive
pulse arrives too soon to let this happen.
But the gate is off.
John